If you are referring to the default 9 lightbox images within the Jammin Template and linked to via the navigation menu of Gallery, these are basic sliders and created directly within Webplus and doesn't use any third party javescript code like some of the older templates. You should be able to find help with regards to sliders and how they work using the help section of Webplus or via Serif's own tutorials on sliders found here - http://www.serif.com/appresources/WPX8/Help/en-gb/webplus.htm (click on Interactive Objects/Sliders) or for more video and text tutorials, see here - https://community.serif.com/tutorials/1/tags/sliders

Your best option however when working with things in Webplus is to open the Objects tab on the right hand side of the work window, here you will find "everything" associated with any given page and because the Jammin Template is a single paged parallax site, you will only need to open the template and then open the objects tab to see everything. Any object that has multiple factors or elements will have a plus sign at the side of it, clicking this will expand the grouped object and show you more elements within say a slider such as the linked images or hotspots that control hyperlinks.

To change a lightbox image, first locate the slider, click the + sign to expand and then locate and click something like "hotspot to file: placeholder.jpg - When you select this option in the objects window, you will see a globe icon (hyperlink) in the main work window and you can then change the hyperlink from here and point it to another file (your new image)

That's about the easiest way I can explain it and hopefully it should get you to where you want to be